This invention relates to a device for the transfer of exhaust gas from the exhaust collector of a supercharged internal combustion engine to the inlet conduit thereof, comprising a supercharge unit with a turbine connected to the exhaust collector, the wheel of which turbine being rotatable by means of flowing exhaust gasses from the exhaust collector and drivingly connected with an impeller of a compressor provided in the inlet conduit, which compressor is connected to on one hand an external inlet conduit part for sucking in air from outside by means of the impeller, and on the other hand to said inlet conduit for feeding of pressurized charging air to the engine, and a transfer conduit extending between the exhaust collector and the inlet conduit for, when necessary, transferring exhaust gasses from the exhaust collector to the inlet conduit in and for mixing with the charging air before the latter is fed to the engine, a venturi device being provided in the inlet conduit, of the type that comprises a canal that is delimited by on one hand a waist-like section with a smallest cross-sectional area, and on the other hand two sections with a tapering shape, viz. a converging inlet section located upstream of the waist section and a diverging outlet section functioning as a diffusor located downstream of the same, and the transfer conduit ending in or upstream of the waist section in the canal, so that exhaust gasses from the exhaust collector be sucked into the pressurized charging air in the inlet conduit, viz. by the venturi effect.
The recirculation of exhaust gas or the so called EGR (=Exhaust Gas Recirculation), is applied in order to reduce the concentration of nitrogen oxides (Nox) in exhaust gasses from internal combustion engines, such as Diesel engines. Here, the returned exhaust gasses or EGR gasses function in a way that the combustion temperature in the cylinders of the engine is lowered, whereby the amount of nitrogen in the inlet air that may be transformed into nitrogen oxides, is reduced. At turbo-supercharged Diesel engines, which are the most common type of heavy Diesel engines, the EGR systems may be divided into two main categories, viz. so called long way systems and short way systems, respectively. In the long way systems, the necessary amount of EGR gasses is taken out downstream of the turbo unit turbine and is introduced upstream of the unit compressor. In the short way systems, the EGR gasses are taken out upstream of the turbine, viz. via a transfer conduit that is connected to the inlet side of the engine at a point located somewhere downstream of the compressor. In the transfer conduit is a control valve for the purpose of regulating the amount of drawn-off EGR gas, normally in dependence on parameters that represent the operation state of the engine. One problem for specifically short way systems is that if the turbo supercharge is made effective, the pressure of the charging air in the inlet conduit will become higher than the gas pressure in the exhaust H collector. This involves that some form of pressure-increasing or sucking means has to be used in order to force the EGR gasses into the inlet conduit. A previously applied method of sucking the EGR gasses into the inlet conduit is to provide a venturi device of the type as defined in the preamble. In the narrow, waist-like section in a venturi canal, the velocity of a passing gas becomes high, at the same time as the statical gas pressure becomes low, which involves that the EGR gasses may be sucked in from the exhaust collector via the transfer conduit. The outlet section situated downstream of the waist section diverges in the downstream direction and serves as a diffusor, which slows down the gas and increases the pressure in it. In this way, the charging air recovers a certain part of its original pressure after having been mixed with EGR gasses and passed the waist section.
Another way of forcing the EGR gasses into the charging air is to simply pump in the EGR gasses by means of a pump.
A particular problem in connection with the use of a venturi device for sucking in the EGR gasses is that the charging air flow and the pressure conditions between the exhaust collector and the inlet conduit vary in dependence of the different load points of the internal combustion engine. One way of solving specifically this problem is to construct the venturi canal with a narrowest waist section that is sufficiently small to be capable of sucking the required amount of EGR gasses in all load points. In order to restrict the amount of EGR gas in all other load points, a certain amount of charging air is led past or outside the venturi device via a special bypass conduit (cf., e.g., WO 96/32583). However, a disadvantage with such a bypass of the charging air is that the pressure recovering-is not achieved and that the venturi device gets a bad efficiency.
Another solution of the problem with varying load conditions is to make the venturi device with an elastically deformable canal wall in the region of the narrow waist section, at same time as the EGR gasses are led into the center of the venturi canal, more specifically via a fixedly arranged injector tube. This problem solution has been published under the denomination VARIVENT and is based on the use of rubber or another elastic material in the canal wall in connection with the waist section of the canal, whose cross-sectional area may be varied by applying varying external forces upon the elastic canal wall, e.g., by means of compressed air. However, in practice, a construction of this type becomes complicated and costly, and the rubber material has a limited life in the tough environment in which the canal wall steadily works, due to alternating contractions and expansions.
The present invention aims at removing the above-mentioned inconveniences of previously known devices for EGR gas transfer and creating an improved transfer device. A primary object of the invention is to create a transfer device with a venturi device that is constructively simple and has a long-term resistance against the gas environment in which the device works. Another object is to provide a device that functions in a reliable way and that makes possible a fine control of the amounts of EGR gasses in the charging air supplied to the engine, also at quickly changing load conditions for the engine. It is also an object to create a gas transfer device in which the venturi device per se is compact and requires a small space for its accomodation.
In accordance with the invention, at least the primary object is achieved by means of the features that are defined in the characterizing clause of claim 1. Furthermore, advantageous embodiments of the invention are defined in the dependent claims.
Further Elucidation of the Prior Art
In U.S. Pat. No. 5,333,456 an exhaust gas transfer device is disclosed with a venturi device in the inlet conduit for charging air between a compressor of a supercharge unit and the engine, the supply of EGR gasses to said conduit being controlled by means of an axially movable control means. However, in this case the control means has the shape of a valve cone applied on a bar, arranged in a constriction in the transfer conduit for the EGR gasses, these EGR gasses being introduced radially into the main inlet conduit for the charging air, more specifically at a point situated downstream of the waist section of the venturi device, i.e., in the diffusor part of the venturi device.